ukify: fix zboot parsing with zstd

The header starts with 'zstd', not 'zstd22':

$ ukify build --linux vmlinuz-6.13+unreleased-cloud-arm64 --initrd /boot/initrd.img-6.12.12-amd64 --output uki
Kernel version not specified, starting autodetection 😖.
Real-Mode Kernel Header magic not found
+ readelf --notes vmlinuz-6.13+unreleased-cloud-arm64
readelf: Error: Not an ELF file - it has the wrong magic bytes at the start
Traceback (most recent call last):
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 2510, in <module>
    main()
    ~~~~^^
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 2499, in main
    make_uki(opts)
    ~~~~~~~~^^^^^^
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 1328, in make_uki
    opts.uname = Uname.scrape(linux, opts=opts)
                 ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 384, in scrape
    version = func(filename, opts=opts)
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 374, in scrape_generic
    text = maybe_decompress(filename)
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 221, in maybe_decompress
    return get_zboot_kernel(f)
  File "/home/bluca/git/systemd/src/ukify/ukify.py", line 201, in get_zboot_kernel
    raise NotImplementedError(f'unknown compressed type: {comp_type!r}')
NotImplementedError: unknown compressed type: b'zstd\x00\x00'

src/ukify/ukify.py

@@ -194,7 +194,7 @@ def get_zboot_kernel(f: IO[bytes]) -> bytes:
         raise NotImplementedError('lzo decompression not implemented')
     elif comp_type.startswith(b'xzkern'):
         raise NotImplementedError('xzkern decompression not implemented')
-    elif comp_type.startswith(b'zstd22'):
+    elif comp_type.startswith(b'zstd'):
         zstd = try_import('zstandard')
         return cast(bytes, zstd.ZstdDecompressor().stream_reader(f.read(size)).read())